1. Field of the Invention
The present invention relates generally to optical testing of light-emitting devices. More specifically, the invention relates to a test apparatus which may be used, in conjunction with automatic test equipment or other equipment, to optically test light emitting diodes (LEDs) or other light-emitting components, in a wide variety of test environments and to the degree necessary pertinent to the type(s) of faults encountered.
2. Background Information
Automatic test equipment (ATE) is typically used to test the electrical functions of complex devices such as, for example, printed circuit boards which may contain a variety of discrete components, integrated circuits and other devices. Printed circuit boards often include light-emitting components, such as LEDs, which must be tested both electrically and optically. Optical testing may involve a simple on/off test (i.e., determining whether a light emitting component is actually illuminated when it should be) or more sophisticated tests such as whether the emitted light exceeds a minimum acceptable intensity; whether the intensities among several components are sufficiently uniform; the color of the emitted light; whether the color of the emitted light matches a predetermined reference, and the like, depending on the commonly encountered faults/failures at any given stage in the manufacture of products using LEDs.
Although an LED is a relatively simple device in itself, the environments in which LEDs require optical testing vary widely. For example, LEDs which require optical testing may exist in a laboratory environment in which there is no impediment to gaining physical access to the LEDs and the amount of time for testing is not critical. In contrast, in other test environments such as mass-production components, assembled printed circuit boards or embedded finished products, there is often extremely limited physical access to the LEDs. Further, the degree of optical testing required and the limited time available for testing often present significant constraints on an optical test solution.
Another constraint on an optical test solution arises from the wide range of luminous intensities produced by present day LEDs. LEDs which are commercially available today typically have luminous intensities ranging from one or two millicandelas (mcd) to thousands of mcd (i.e., a range of four orders of magnitude). Moreover, very low light level illuminated switch button icons used in automobile and aircraft control panels for night-time viewing are typically in the 10 to 200 microcandela (μcd) range. Consequently, an optical test solution must provide a very wide dynamic range of sensitivity in order to accurately optically test such components.
Another significant constraint on an optical test solution is the high density of closely spaced components found on assembled printed circuit boards. In particular, commonly used surface mount LEDs may be mounted on 0.050 inch (1.27 mm) centers. Such close component spacing presents a challenge in terms of preventing interference from light emitted by LEDs that are adjacent to a particular LED under test.
Yet another significant constraint on an optical test solution is the supply voltage necessary to power the sensor. For example, in automotive applications, the standard supply voltage is 12 volts DC. In industrial and avionics applications, the standard supply voltage is 24 and 28 volts DC, respectively. For consumer electronics applications, the standard supply voltage is 5 volts DC. Thus, a sensor intended for consumer electronics applications and designed to operate from a supply of 5 volts DC is likely to be seriously, if not irreparably, damaged should it be inadvertently exposed to the much higher supply voltages used in other applications, or the polarity of the supply reversed.
In addition, an optical test should generate output signals which not only represent the results of the test but which are compatible with ATE or data collection equipment as may be required by a particular application.